Portable devices are becoming more common in every aspect of our lives and affect how we interact with our music. For example, many consumers listen to their music from portable audio players, such as MP3 players or their cellular phones. These portable devices become easier and more enjoyable to use when the portable devices shrink in size while offering the same capabilities and when the portable devices can last longer from a single battery charge. To support longer lasting devices, the operating voltage of the components inside the portable device is often reduced to reduce power consumption by these components. However, a reduction in operating voltage affects the sound output of the portable device, because the volume level of an audio signal output to a speaker is proportional to the output voltage. Thus, boost converters have been used to increase operating voltages for select components within a portable device that benefit from higher voltages, such as the audio amplifier that needs high voltages to generate high volumes.
FIG. 1 is a conventional audio amplifier with a boost converter. An audio amplifier 100 for a mobile device includes a battery supply 102 that provides a voltage, Vp, to a boost converter 104. The boost converter 104 increases the voltage Vp to a boost voltage, Vbst. A speaker amplifier 108 receives the boost voltage, Vbst, and an analog audio signal, sig, from a digital-to-analog converter (DAC) 106. The speaker amplifier 108 increases the low power signal of the audio signal, sig, with power received from the boost converter 104 to generate a signal to drive a speaker 110. The boost voltage, Vbst, is higher than the supply voltage, Vp, which allows the speaker amplifier 108 to provide louder volume ranges through the speaker 110 than possible with only the supply voltage, Vp.
However, providing the boost voltage, Vbst, to the speaker amplifier 108 has drawbacks, such as increased power consumption. That is, because the speaker amplifier 108 is receiving the boost voltage, Vbst, even if volume levels are low, then the speaker amplifier 108 is wasting power. This wasted power shortens the operating time of a portable device between charges and hurts the user's experience with the portable device. Additional inefficiencies exist, such as undesirable power consumption and transients that occur when the speaker 110 is connected and disconnected from the speaker amplifier 108 while the amplifier 108 is receiving the boosted voltage, Vbst. Further, the increased power consumption by the speaker amplifier 108 may cause thermal management problems. Particularly as portable devices shrink in size and have less space for heat dissipation, the speaker amplifier 108 may be constrained in volume ranges it can provide by heat buildup in the speaker amplifier 108 or the boost converter 104. Each of these shortcomings is due to the static nature of the boost voltage, Vbst, provided to the speaker amplifier 108.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved audio amplifiers, particularly for consumer-level devices. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art.